I. Field of the Invention
The present invention relates generally to electronic sensor and, more particularly, to a sensor for detecting the air/fuel ratio in the exhaust gas of an internal combustion engine.
II. Description of the Prior Art
The operational characteristics of an internal combustion engine, such as those used in automotive engines, are strongly dependent upon the air/fuel (A/F) ratio of the A/F mixture. These operation characteristics are illustrated in FIG. 1 where the air excess ration .lambda. is defined by the following equation: ##EQU1##
Where (AF).sub.st is the stoichiometric A/F ratio.
Thus, as shown in FIG. 1, a lean A/F mixture (.lambda..degree.1.0) results in lower fuel consumption rates, lower engine power Band higher NO.sub.K engine emissions than either a rich or stoichiometric A/F mixture. Conversely, a rich A/F mixture (.lambda.&lt;1.0) results in greater engine power and higher engine fuel consumption. Simultaneously, a rich A/F mixture results in higher exhaust emissions of carbon monoxide (CO) and lower emissions of NO.sub.K.
During the normal operation of an automotive engine, the A/F ratio will vary between rich and lean depending upon the demanded load conditions which would include, for example, acceleration, deceleration, cruise and idle engine conditions. Ideally, the fuel management system for the engine adjusts the A/F ratio of the fuel mixture for optimal engine performance.
With reference now to FIG. 2, the relationship between the air excess ration (.lambda.) and the exhaust emissions from the internal combustion engine are illustrated. As is evident from FIG. 2, for rich fuel mixtures (.lambda.&lt;1.10) the exhaust emissions contain very little oxygen (O.sub.2) since the available oxygen has combusted with the fuel. Conversely, the oxygen content for the exhaust emissions increases for a lean A/F mixture (.lambda.&gt;1.0) since there is insufficient fuel to combust with the oxygen.
There have been previously known A/F ratio sensors which detect the oxygen content in the exhaust from the engine and, from the oxygen content in the exhaust emissions, determine the A/F ratio for the engine. These previously known sensors typically comprise a solid electrolyte having one side open to a diffusion layer and a second side open to atmospheric air which has a constant oxygen content. The diffusion layer itself contains porous apertures that are exposed to the exhaust gases from the engine. An electronic circuit is then employed to detect both the magnitude and direction of the oxygen ion flow through the solid electrolyte and to provide an electrical output signal representative of this ionic transfer. This output signal is then utilized as an input signal to the fuel management system for the engine.
A primary disadvantage of this previously known type of A/F ratio sensor is that, after extended use, the aperture between the solid electrolyte of the sensor and the exhaust gas emissions becomes contaminated and clogged by particles in the exhaust gas flow. Once the aperture to the solid electrolytic cell is clogged, the sensor may eventually fail and must be replaced.